subsurf splits the mirrored model into two objects
A couple of things could be effecting your model.


Internal faces cause the subsurf modifier to "pull in". To fix this you need to delete the faces that will become internal faces after the mirror modifier duplicates your mesh.





The order that the modifiers are listed effects how they interact. Having the subsurf before the mirror can cause the edges in the centre of the mesh to pull away before the mirror duplicates the mesh. To fix this you need to have the subsurf under the mirror modifier. You can use the up and down arrows on the modifiers to re-order them.
If your subsurf modifier is after the mirror and the edges still shrink away from the centre of your object you need to enable merge in the mirror modifier. Enabling clipping in the mirror modifier can also help to get the centre vertices aligned so that they merge.